The present invention relates to systems and methods for monitoring fetal heart beat events, for example, for diagnosing fetal well being.
Doppler ultrasound systems monitor motions of the fetal heart by processing ultrasonic echo signals reflected from heart walls and valves to detect the Doppler shift of the echoes induced by these moving tissue surfaces.
The echo signals, however, include large Doppler shifted components produced by motions of other tissue surfaces. For example, fetal motions as well as motions of maternal tissue surfaces introduce large artefacts in the echo signals which greatly complicate the task of deriving useful information of fetal heart activity therefrom.
Various techniques have been employed for extracting information of heart activity from the echo signals. For example, directional Doppler systems segregate the echo signals into towards and away motion components for deriving simplified signals which may be peak detected to provide generally reliable indications of fetal heart activity. However, it is thus necessary to select one of the towards or away echo components for producing a heart beat event signal and the selection of a component representing noise can result in a failure to detect one or more subsequent heart beat events. U.S. Pat. No. 4,143,650 to Hatke describes a typical directional Doppler fetal heart beat monitoring system.
U.S. Pat. No. 3,982,528 to Phillipps, assigned to the assignee of the present application, describes a system for monitoring fetal heart beat events wherein fetal heart signals from a mechanical energy transducer, such as an ultrasound transducer, are correlated against a sample signal derived from such signals corresponding to one or more previous fetal heart cycles to produce a correlation signal. A peak of the correlation signal is detected and the system produces an event signal if a new peak is not subsequently detected within a fixed delay time period thereafter. This permits the system to trigger one and only one event signal for each fetal heart cycle provided that the fetal heart rate remains within certain limits, that is, between approximately 105 to 210 beats per minute.
The fetal heart rate, however, varies within a range of about 50 beats per minute to over 200 beats per minute. Consequently, when the fetal heart rate is in the range of 50-105 beats per minute, prior art techniques, such as disclosed in Phillipps, often detect the fetal heart rate at twice the real rate. More particularly, when the ventricular ejection time approaches half the fetal heart period a fetal heart signal having two heart sounds that are nearly equidistant from each other occurs. Inasmuch as these two heart sounds when processed by systems such as Phillipps result in two correlation peaks separated in time by more than the fixed delay period, double counting of the fetal heart rate occurs.
A further complication in the use of correlation techniques in fetal heart monitoring is introduced by the nature of the correlation function which is directly related in amplitude to the energy content of the input signals. Should a large artefact component be present in the input signals, for example, due to a kick by the fetus, the correlation function can jump in amplitude causing an overload and a loss of important information.
A still further complication is introduced where a temporary loss of the signal or the presence of a spurious peak in the correlation function creates a distortion in the monitored heart rate.